It might not be the latch, which is more of a squeal on our rig.
Our 2004 Horizon suffered this problem, as did our 2001 Horizon. For us, the issue on both was that the door is made of 2 layers, an inner "sub skin" and the exterior "skin" that we see. The problem occurs at highway speeds and can be exacerbated by strong or buffeting winds across the front right corner of the rig. What we learned is that the outer skin of the door flexes and rattles against the inner "sub skin".
Our solution was to dis-assemble the door and insert a buffer (in this case, indoor/outdoor carpet) between the 2 surfaces. Worked like a charm after much consternation on the 2001 and its wasn't a fight to get Winnie to do it in 2004 on the current one. Quiet ever since. It also adds some silence to the door, quieting the passenger area a little. (side note: I am about to cover the entire underside of the front cockpit area exposed metal with reflectix insulation to add R value and quiet!)
The outer skin of the lower part of door is riveted on, and its easier if you remove the window glass first. It sounds scary, but it really wasn't that bad. (I was naughty and watched the tech do it). I made sure that they re-assembled it with copious Sikaflex, and rivets.
Hope this helps. Feel free to contact me with any questions.